Smoke domain causes artefacts in non-density areas
Without a screenshot it's hard to tell, but I often see artifacts in smoke simulations even in tutorials because of too low transparency settings. Try to increase the Transparency value under Render Settings &gt; Light Paths &gt; Max Bounces. Maybe 32 or 64 or even 128. Important: Transparency, not Transmission (image is from a different question, the red framed part is important).